1. Field of the Invention
The present invention relates to a boundary acoustic wave resonator in which reflectors are disposed on either side of an IDT electrode in a boundary acoustic wave propagating direction and a ladder filter including the boundary acoustic wave resonator, and, more particularly, to a boundary acoustic wave resonator in which apodization weighting is performed on an IDT electrode, and also relates to a ladder filter including the boundary acoustic wave resonator.
2. Description of the Related Art
Surface acoustic wave resonators are widely used in communication apparatuses, such as mobile telephones, to form resonators or filters.
For example, Japanese Unexamined Patent Application Publication No. 2000-286663 discloses a surface acoustic wave resonator 1001 illustrated in FIG. 12. In the surface acoustic wave resonator 1001, an electrode structure illustrated in Fig. is provided on a piezoelectric substrate. The surface acoustic wave resonator 1001 uses a Love wave having an electrochemical coefficient k2 larger than that of a Rayleigh wave.
In the surface acoustic wave resonator 1001, an IDT electrode 1002 is formed on the piezoelectric substrate. The IDT electrode 1002 includes a busbar 1003 and a busbar 1004 facing the busbar 1003. The busbar 1003 includes a busbar portion 1003a extending in a direction inclined at an angle of θ with respect to a surface wave propagating direction and a busbar portion 1003b that extends in a direction inclined at an angle of −θ with respect to a surface acoustic wave propagating direction and is connected to the busbar portion 1003a. 
Similarly, The second busbar 1004 includes a busbar portion 1004a extending in a direction inclined at an angle of −θ with respect to the surface acoustic wave propagating direction and a busbar portion 1004b that is connected to the busbar portion 1004a and extends in a direction inclined at an angle of θ with respect to the surface acoustic wave propagating direction.
The busbar portions 1003a, 1003b, 1004a, and 1004b form a substantially rhombus shape.
A plurality of electrode fingers 1005 extend from the busbar portions 1003a and 1003b toward the busbar portions 1004a and 1004b. Dummy electrodes 1006 are disposed to face the ends of the electrode fingers 1005 with gaps therebetween. One end of each of the dummy electrodes 1006 is connected to the second busbar 1004 and the other end thereof faces the electrode fingers 1005 with the above-described gaps therebetween.
A plurality of electrode fingers 1007 is similarly disposed. One end of each of the electrode fingers 1007 is connected to the second busbar 1004 and the other end thereof extends toward the first busbar 1003. Dummy electrodes 1008 are disposed to face the ends of the electrode fingers 1007 with gaps therebetween in the length direction of the electrode fingers. One end of each of the dummy electrodes 1008 is connected to the first busbar 1003 and the other end thereof faces the electrode fingers 1007 with the above-described gaps therebetween.
The electrode fingers 1005 and the electrode fingers 1007 are alternately disposed in the surface acoustic wave propagating direction. Apodization weighting is performed on the IDT electrode 1002. The apodization weighting provides a maximum intersecting width at the center of the IDT electrode in the surface wave propagating direction. The intersecting width decreases as the distance from the center in the surface wave propagating direction increases.
In the surface acoustic wave resonator 1001, the minimum intersecting width is zero. There are areas at the ends of the IDT electrode in the surface acoustic wave propagating direction in which only the dummy electrodes 1006 and 1008 are present.
A feature of the surface acoustic wave resonator 1001 is that apodization weighting is performed as described above and an envelope A obtained by the apodization weighting is parallel to the inner sides of the busbar portions 1003a, 1003b, 1004a, and 1004b. That is, the inner sides of the busbar portions 1003a to 1004b are arranged so that they are parallel to the envelope. The inner sides of the busbar portions 1003a to 1004b are inclined at an angle of θ or −θ with respect to the surface wave propagating direction. For this reason, resonance in an anharmonic higher-order mode rarely occurs. This leads to the suppression of a spurious response. In particular, as disclosed in Japanese Unexamined Patent Application Publication No. 2000-286663, portions between the above-described envelope and the inner sides of the busbars extending in parallel with the envelope function as reflectors. Accordingly, for example, as represented by a straight line Lo in FIG. 13, an excited surface acoustic wave crosses, for example, five electrode fingers before reaching the inner side of the busbar portion 1003b. Since these five electrode fingers function as reflectors, a spurious response can be effectively suppressed. This leads to size reduction.
On the other hand, since a space above an IDT electrode is not required in a boundary acoustic wave resonator, boundary acoustic wave resonators are attracting attention. As in surface acoustic wave resonators, it is also necessary to suppress a spurious response in boundary acoustic wave resonators.
Surface acoustic wave resonators are used to form oscillation circuits, filters, and other devices. To form a filter circuit, a plurality of surface acoustic wave resonators is typically connected. For example, in a ladder filter including a plurality of surface acoustic wave resonators, at least one surface acoustic wave resonator is connected to a series arm and at least one surface acoustic wave resonator is connected to a parallel arm. In a ladder filter, attenuation is not sufficiently increased in a band higher than a passband when the impedance of a series arm resonator at an anti-resonant frequency is not sufficiently high.
In a parallel arm resonator, an insertion loss may be increased in a passband when the impedance thereof at an anti-resonant frequency is not sufficiently high.
However, in the surface acoustic wave resonator disclosed in Japanese Unexamined Patent Application Publication No. 2000-286663, the impedance thereof at an anti-resonant frequency is not sufficiently high. A return loss is increased in a frequency band higher than the anti-resonant frequency, for example, at a frequency that is approximately 1.003 times the anti-resonant frequency. For this reason, an insertion loss is increased in a higher portion of the passband of a ladder filter including the surface acoustic wave resonator as a parallel arm resonator.
On the other hand, as in surface acoustic wave resonators, in boundary acoustic wave resonators, it is also necessary to suppress a spurious response caused by resonance in a higher-order mode and to obtain a high impedance at an anti-resonant frequency when such a boundary acoustic wave resonator is used in a ladder filter.